When I was in your shoes I made it clear to M that I would help in only one of these ways:
Make sure she got a meal if she was hungry.
Make sure she got a ride to a rehab she had arranged.
Make sure she got a ride to the ER if she needed to go.
I also made it clear to her that THAT WAS ALL I would do for her. That clearly communicates that you care for this persons well being and sets expectations of what you're willing to do to help. Being an ear to bend during yet another fit of suicidal depression was not something I was willing to do. BTW, I never got a request for what I was willing to do. And if you ever suspect he is suicidal you can call 911 and report his condition and location, they will send help, maybe not the help he wants but the help he needs.